In graphic arts technology, a number of well-established printing processes utilize image carriers with a three-dimensional (3D) representation of data, the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. A relief plate includes raised relief features, which are raised above the plate floor. It is the raised features that accept and transfer ink to the substrate. In a traditional flexographic prepress process with chemical etching, there is no possibility of fine control of relief properties other than the relief depth.
Flexographic printing uses a flexible relief plate 20 to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. A simplified diagram of a flexographic printing press 30 is shown in FIG. 1. Ink 10 in a fountain pan 26 is taken up by a rubber fountain roller 12 and transferred to the surface of the anilox roller 14. The surface of the anilox roller 14 is composed of an array of indented cells that allow careful metering of the ink volume. A doctor blade 16 removes any excess ink 10 from the anilox roller 14 before the ink 10 is transferred to the printing plate cylinder 18. Mounted on the plate cylinder 18 is a flexographic printing plate 20, which can also be referred to as a flexible relief plate. The final step transfers the ink 10 from the plate 20 to substrate 22, with impression cylinder 24 supplying support for the substrate 22.
The process used to produce an image on a flexible relief plate 20 typically includes the following steps:
1. Expose the back of the plate to UV light;
2. Expose an intermediate film to the desired image;
3. Laminate the film to the top of the plate;
4. Expose the plate through the film using UV light;
5. Remove the film;
6. Use a solvent to wash away the unexposed plate material;
7. Apply additional exposure to harden the plate;
8. Dry the plate to remove as much of the solvent as possible.
The back exposure in step 1 is used to establish the floor 220 of the plate 20. The intensity of the exposure decreases as the illumination penetrates the plate because of absorption in the plate material. Once the intensity drops below a threshold value, there is insufficient cross linking in the polymer comprising the plate, and the remaining under-exposed polymer can be washed away. This is typically the top 0.5 mm of the plate. To form the relief, the front of the plate is exposed through an image layer with enough intensity so that sufficient cross-linking occurs all the way down to the plate floor 220.
For every opening in the image layer, a cone of UV light (typically with an angle of about 40 degrees from a normal to the plane) propagates through the plate 20 forming cone-shaped relief dots. A cross section of an exemplary plate 20 is shown in FIG. 2. The following features are depicted in the cross section: a solid area raised feature 204; an isolated dot raise feature 208; and a raised feature 212 including an array of closely spaced dots created by a halftone screen. The height of the plate relief above the plate floor 220 is shown by relief depth 216.
Ink uniformity and density can be improved if a surface pattern or surface texture is applied to the flat tops of the relief pattern as shown in the FIG. 3, which compares a no surface texture pattern 302, a conventional plate cell pattern 303 and a checkerboard surface texture 304. The checkerboard surface texture 304 in this example is provided by a Kodak DigiCap NX Screening process which forms a stretched checkerboard pattern 304 composed of 5×10 micron rectangles. This texture pattern has been found to work well for process inks printed on a paper substrate.
Such a fine texture pattern has an additional advantage in that it allows the edges of printing features to be well defined. The pattern does have its limits. When printing on plastic substrates, voids can appear in large features due to air entrapment. The pattern can also perform poorly if large volumes of ink need to be transferred to the substrate 22. To eliminate these problems, a coarser pattern is required. However, a coarser pattern will compromise edge definition.
In flexographic printing, large solid areas of a relief pattern can suffer from a number of artifacts. The ink can deposit unevenly, resulting in a reduction in ink density, and in the solid area having a mottled appearance. Ink can be squeezed off the relief feature near edges resulting in low ink density just inside the edge and high density just outside the edge. Air bubbles trapped between the plate 20 and substrate 22 can cause voids to appear at the trailing edge of large features.
Prior art exists to mitigate some these problems. For the case of voids due to air bubbles, U.S. Patent Application Publication 2010/0224091 to G. Zwadlo, entitled “Trailing edge pattern for relief plate feature,” describes a method that reduces voids by forming sunken patterns in the trailing edge of large print features.
U.S. Patent Application Publication 2016/0221379 to Y. Namba et al., entitled “Flexo printing plate,” attempts to deal with the voids by introducing small depressions in the large solid relief regions by having the density of the small depressions decrease with distance from the edge of the relief.
A preferred method combines a fine texture pattern at the edge of printing features with a coarser pattern in the interior of features as described in commonly-assigned U.S. Pat. No. 9,235,126 to R. Bielak et al., entitled “Flexographic surface patterns,” which is incorporated herein by reference This approach is illustrated in the textured image patterns 400 of FIGS. 4 and 5. In FIG. 4, a fine texture pattern 404 is applied near the edges of the relief features 402 and a coarse texture pattern 408 is applied to the interior of the relief features 402. FIG. 5 is similar to FIG. 4 except that the coarse texture pattern 408 is coarser than that of FIG. 4. A weakness of this method is that the interior pattern cannot be simultaneously optimized for both uniform ink deposition and resistance to voids at the trailing edge of large relief features 402.
There remains a need for a method for introducing texture patterns to relief features in a flexographic printing plate which is simultaneously optimized for both uniform ink deposition and resistance to voids at the trailing edge of large relief features.